What is this disk geometry thing

From Soekris Info Wiki

If you burn a CF card on your PC, it will get its disk geometry values from your PC's
BIOS. These are not necessarily the same values the Soekris comBIOS sees. If you are
running a boot loader which uses absolute sector numbers (Lilo), you'll probably never
notice this, as such a boot loader can handle the anomalous geometry. However, if
you're running a boot loader which is FS-aware (most modern BLs), you might end up
with a system that apparently refuses to boot.

The solution is to pass fdisk the geometry data (C/H/S values) as displayed in the
serial console when you boot up the Soekris, when burning the CF. After that, things
should run without a problem.

To clarify: when you boot your Soekris and haven't set it to be quiet, you see something like: